The present invention relates to cooling flow transition inserts for transitioning gas flow in stator core cooling ducts of electrical machines and particularly relates to cooling flow transition inserts and methods of cooling the stator core which reduce the total pressure losses in the cooling ducts.
In many electrical machines such as generators, particularly large-scale electrical machines, the stator cores must be cooled. It is a common practice to direct cooling gas for flow through a series of generally radially extending ducts that connect between the inner diameter of the stator core and its outer diameter. The ducts extend between sets or groups of laminations lying normal to the machine axis and at axially spaced locations along the stator core. In certain electrical machines, the pressure drop through the stator core can become a limiting aspect of the overall machine cooling system. This is particularly true in air-cooled machines in which air handling is key to the success of the machine.
Cooling gas in these machines generally flows radially inwardly or outwardly and encounters along the flow paths abrupt contractions or expansions, particularly at the bottoms of the stator slots. The stator slots receive the axially extending armature bars about which flow the cooling gas in the ducts. The armature bars pass through the gaps between the groups or sets of laminations in the cooling ducts. A common and current configuration of the armature bars in the slots of large electrical machines generally affords an abrupt flow transition through 90xc2x0 bends in the flow path caused by the existence of the armature bars in the ducts. These abrupt transitions cause pressure losses in addition to those occurring in the ducts when the cooling gas enters and exits the duct and as it flows along the duct.
With the exception of the friction loss, each of the pressure changes may represent a local increase or decrease in the static pressure. All, however, entail a loss in total pressure. Thus, a portion of system energy is irreversibly converted to heat and appears as a reduction in the efficiency of the overall machine. Consequently, there is a need for apparatus and methods to improve the flow of the cooling gas to minimize the flow losses.
In accordance with a preferred embodiment of the present invention, there is provided a flow transition insert in each cooling duct that enables the flow path to gradually expand and thereby reduce the total pressure loss in the cooling ducts of the stator core. The radially extending cooling ducts which conduct cooling flow both radially inwardly and outwardly at different axial locations along the stator core include first and second radially extending cooling path sections. The first flow path section of each stator slot extends from the radially innermost portion of the core to the radially outermost portion of the armature bars and the second flow path section extends radially outwardly of the first flow path section. Thus, there is a juncture between the first and second flow path sections at the radially outermost portion of the armature bars. Where that juncture typically has an abrupt 90xc2x0 bend in current electrical machines, a transition insert is provided in the duct at the juncture of those sections and extends into the second flow path section to gradually transition the flow thereby reducing the pressure drop. The transition inserts per se may have linear or curvilinear surfaces in part defining the flow path. It will be appreciated that these surfaces permit the flow to expand or contract gradually and consequently reduce pressure losses at that juncture.
In a preferred embodiment according to the present invention, there is provided an electrical machine comprising a stator core including a plurality of axially spaced stator core laminations extending normal to an axis of the machine, armature bars passing generally axially through armature bar slots in the laminations, generally radially extending ducts defining flow paths for flowing a cooling gas through the stator core, the ducts extending between a pair of axially spaced laminations and about the axially extending armature bars, the armature bars forming with the ducts steps in the flow paths and defining first and second radially spaced flow path sections of the ducts, the first flow path sections extending generally radially alongside the armature bars and the second flow path section lying radially outward of the armature bars and the first flow path section and a gas flow transition element in each flow path radially outwardly of the armature bars having a flow surface for transitioning the flow along the duct between the first and second flow path sections thereof to minimize cooling gas flow losses associated with the step.
In a further preferred embodiment according to the present invention, there is provided in an electrical machine having a stator core including a plurality of stator core laminations, armature bars passing through armature slots in the laminations and through generally radially extending ducts defining flow paths for flowing a cooling gas through the stator core, the armature bars forming steps in the flow paths and defining first and second radially spaced flow path sections of the ducts, a method of cooling the stator core comprising the step of disposing a flow transition element in the second gas path section of each duct having a transition flow surface for gradually transitioning the flow between the first and second flow path sections to minimize cooling gas flow losses associated with said step.
FIG. 1 is a fragmentary schematic side elevational view of an end portion of an electrical machine illustrating stator core cooling ducts forming part of the present invention;
FIG. 1 is an enlarged cross-sectional view of the stator ducts in accordance with a prior cooling duct design;
FIG. 3 is a view similar to FIG. 2 illustrating the transition inserts in the stator core cooling ducts in accordance with a preferred form of the present invention; and
FIGS. 4A-4D illustrate four different forms of transition inserts.